Unfortunately I've got an issue right off the bat, I recently bought a 2000 Monte Carlo LS which at the time had working brake lights and this evening a friend of mine told me when he was behind my car the brake lights did not come on. Then I had him set in the car and press on the brakes and sure enough they weren't coming on.

 

Welcome to the forum..

First I would check all the fuses both in the car and under the hood...

do you have a 3rd brake light? and does it still work?
If not...
It may just need a new brake light switch. Its beneath the brake pedal... Maybe it came unplugged?

Do the turn signals still work?

If all the bulbs and fuses are good...

It would guess it maybe a failed multifunction switch...

Pls keep us posted on your progress...
Im sure other members will chime in with more ideas..

 

Probably the turn signal switch, but could be bulbs or the brake switch also